namespace MoreLinq;

using System;
using System.Collections.Generic;

static partial class MoreEnumerable
{
    /// <summary>
    /// Returns all distinct elements of the given source, where "distinctness"
    /// is determined via a projection and the default equality comparer for the projected type.
    /// </summary>
    /// <remarks>
    /// This operator uses deferred execution and streams the results, although
    /// a set of already-seen keys is retained. If a key is seen multiple times,
    /// only the first element with that key is returned.
    /// </remarks>
    /// <typeparam name="TSource">Type of the source sequence</typeparam>
    /// <typeparam name="TKey">Type of the projected element</typeparam>
    /// <param name="source">Source sequence</param>
    /// <param name="keySelector">Projection for determining "distinctness"</param>
    /// <returns>A sequence consisting of distinct elements from the source sequence,
    /// comparing them by the specified key projection.</returns>

#if NET6_0_OR_GREATER
    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector)
#else
        public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> keySelector)
#endif
    {
            return DistinctBy(source, keySelector, null);
        }

    /// <summary>
    /// Returns all distinct elements of the given source, where "distinctness"
    /// is determined via a projection and the specified comparer for the projected type.
    /// </summary>
    /// <remarks>
    /// This operator uses deferred execution and streams the results, although
    /// a set of already-seen keys is retained. If a key is seen multiple times,
    /// only the first element with that key is returned.
    /// </remarks>
    /// <typeparam name="TSource">Type of the source sequence</typeparam>
    /// <typeparam name="TKey">Type of the projected element</typeparam>
    /// <param name="source">Source sequence</param>
    /// <param name="keySelector">Projection for determining "distinctness"</param>
    /// <param name="comparer">The equality comparer to use to determine whether or not keys are equal.
    /// If null, the default equality comparer for <c>TSource</c> is used.</param>
    /// <returns>A sequence consisting of distinct elements from the source sequence,
    /// comparing them by the specified key projection.</returns>

#if NET6_0_OR_GREATER
    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer)
#else
        public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer)
#endif
    {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (keySelector == null) throw new ArgumentNullException(nameof(keySelector));

            return _(source, keySelector, comparer);

            static IEnumerable<TSource> _(
                IEnumerable<TSource> source,
                Func<TSource, TKey> keySelector,
                IEqualityComparer<TKey>? comparer)
            {
                var knownKeys = new HashSet<TKey>(comparer);
                foreach (var element in source)
                {
                    if (knownKeys.Add(keySelector(element)))
                        yield return element;
                }
            }
        }
}
